While LCD displays offer a compact, lightweight alternative to CRT monitors, there are many applications for which LCD displays are not satisfactory due to a low level of brightness, or more properly, luminance. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light-providing surface positioned behind the LCD for directing light outwards, towards the LCD. The light-providing surface itself provides illumination that is essentially Lambertian, that is, having an essentially constant luminance over a broad range of angles. With the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution toward normal, relative to the display surface. Among proposed solutions for brightness or luminance enhancement for use with LCD displays and with other types of backlit display types are the following:
U.S. Pat. No. 5,592,332 (Nishio et al.) discloses the use of two crossed lenticular lens surfaces for adjusting the angular range of light in an LCD display apparatus;
U.S. Pat. No. 5,611,611 (Ogino et al.) discloses a rear projection display using a combination of Fresnel and lenticular lens sheets for obtaining the desired light divergence and luminance;
U.S. Pat. No. 6,111,696 (Allen et al.) discloses a brightness enhancement film for a display or lighting fixture. With the optical film disclosed in the '696 patent, the surface facing the illumination source is smooth; the opposite surface has a series of structures, such as triangular prisms, for redirecting the illumination angle. The film disclosed in the '696 patent refracts off-axis light to provide a degree of correction for directing light at narrower angles. However, this film design works best for redirecting off-axis light; incident light that is normal to the film surface may be reflected back toward the source, rather than transmitted;
U.S. Pat. No. 5,629,784 (Abileah et al.) discloses various embodiments in which a prism sheet is employed for enhancing brightness, contrast ratio, and color uniformity of an LCD display of the reflective type. In an embodiment disclosed in the '784 patent, the brightness enhancement film similar to that of the '696 patent is arranged with its structured surface facing the source of reflected light for providing improved luminance as well as reduced ambient light effects. Because this component is used with a reflective imaging device, the prism sheet of the '784 disclosure is placed between the viewer and the LCD surface, rather than in the position used for transmissive LCD systems (that is, between the light source and the LCD);
U.S. patent application publication Ser. No. 2001/0053075 (Parker et al.) discloses various types of surface structures used in light redirection films for LCD displays, including prisms and other structures;
U.S. Pat. No. 5,887,964 (Higuchi et al.) discloses a transparent prism sheet having extended prism structures along each surface for improved back-light propagation and luminance in an LCD display. As is noted with respect to the '696 patent mentioned above, much of the on-axis light is reflected rather than transmitted with this arrangement. Relative to the light source, the orientation of the prism sheet in the '964 disclosure is reversed from that used in the '696 disclosure. The arrangement shown in the '964 disclosure is usable only for small, hand-held displays and does not use a Lambertian light source;
U.S. Pat. No. 6,356,391 (Gardiner et al.) discloses a pair of optical turning films for redirecting light in an LCD display, using an array of prisms, where the prisms can have different dimensions;
U.S. Pat. No. 6,280,063 (Fong et al.) discloses a brightness enhancement film with prism structures on one side of the film having blunted or rounded peaks;
U.S. Pat. No. 6,277,471 (Tang) discloses a brightness enhancement film having a plurality of generally triangular prism structures having curved facets;
U.S. Pat. No. 5,917,664 (O'Neill et al.) discloses a brightness enhancement film having “soft” cutoff angles in comparison with conventional film types, thereby mitigating the luminance change as viewing angle increases;
U.S. Pat. No. 5,839,823 (Hou et al.) discloses an illumination system with light recycling for a non-Lambertian source, using an array of microprisms; and,
U.S. Pat. No. 5,396,350 (Beeson et al.) discloses a backlight apparatus with light recycling features, employing an array of microprisms in contact with a light source for light redirection in illumination apparatus where heat may be a problem and where a relatively non-uniform light output is acceptable.
FIG. 1 shows one type of brightness enhancement film 10 for enhancing light provided from a light source 18. Brightness enhancement film 10 has a smooth side 12 facing towards a light guiding plate 14, which contains a reflective surface 19, and rows of prismatic structures 16 facing an LCD component 20. This arrangement, as described in U.S. Pat. Nos. 6,111,696 and 5,629,784 (both listed above), and in 5,944,405 (Takeuchi et al.), generally works well, improving the on-axis luminance by refraction of off-axis light rays and directing a portion of this light closer to the normal optical axis. As FIG. 1 shows, off-axis rays R1 are refracted toward normal. It is instructive to note, however, that, due to total internal reflection (TIR), near-axis light ray R3 can be refracted away from normal at a more extreme angle. In addition, on-axis light ray R4 can actually be reflected back toward light guiding plate 14 for diffusion and reflection from reflective surface 19 rather than directed toward LCD component 20. This refraction of near-axis light and reflection of at least a portion of on-axis light back into light guiding plate 14 acts to adjust illumination luminance with respect to viewing angle, as is described subsequently. By the action of light guiding plate 14 and reflective surface 19, a portion of the light that is reflected back from brightness enhancement film 10 is eventually diffused and again directed outward toward the LCD component at a generally normal angle. There is, of course, some loss of light after multiple reflections, due to inefficiency of reflective surface 19.
The purpose of brightness enhancement film 10, is to redirect the light that is provided over a large angular range from light guiding plate 14, so that the output light it provides to LCD component 20 is generally directed toward normal. By doing this, brightness enhancement film 10 helps to improve display luminance not only when viewed straight-on, at a normal to the display surface, but also when viewed from oblique angles.
As the viewer angle from normal increases, the perceived luminance can diminish significantly beyond a threshold angle. The graph of FIG. 2 shows a luminance curve 26 that depicts the characteristic relationship of luminance to viewer angle when using the brightness enhancement film 10. As expected, luminance peaks at the normal and decreases toward a threshold cutoff angle Ocutoff each side of normal. A slight increase occurs beyond angle Ocutoff; however, this represents wasted light, not readily perceptible to the viewer due to characteristics of the LCD display itself.
With reference to luminance curve 26 in FIG. 2, one characteristic of interest is the overall shape of the curve. The luminance over a range of viewing angles is proportional to the area under the curve for those angles. Typically, the peak luminance values occur at angles near normal, as would be expected. In many applications, it is most beneficial to increase luminance within a small range of viewing angles centered about a normal.
While conventional approaches, such as those noted in the disclosures mentioned hereinabove, provide some measure of brightness enhancement at low viewing angles, these approaches have some shortcomings. Some of the solutions noted above are more effective for redistributing light over a preferred range of angles rather than for redirecting light toward normal for best on-axis viewing. These brightness enhancement film solutions have a directional bias, working best for redirecting light in one direction. For example, a brightness enhancement film may redirect the light path in a width direction relative to the display surface, but have little or no affect on light in the orthogonal length direction. As a result, multiple orthogonally crossed sheets must be overlaid in order to redirect light in different directions, typically used for redirecting light in both horizontal and vertical directions with respect to the display surface. Necessarily, this type of approach is somewhat a compromise; such an approach is not optimal for light in directions diagonal to the two orthogonal axes.
As disclosed above, brightness enhancement articles have been proposed with various types of refractive surface structures formed atop a substrate material, including arrangements employing a plurality of protruding prism shapes, both as matrices of separate prism structures and as elongated prism structures, with the apex of prisms both facing toward and facing away from the light source. For the most part, these films exhibit directional bias, requiring the use of multiple sheets in practical applications.
A number of the patent disclosures have disclosed use of Total Internal Reflection (TIR) effects for redirecting light within prism structures having tilted side walls. For example:
U.S. Pat. Nos. 5,739,931 and 5,598,281 to Zimmerman et al. disclose illumination apparatus for backlighting, using arrays of microprisms and tapered optical structures;
U.S. Pat. No. 5,761,355 to Kuper et al. discloses arrays for use in area lighting applications, wherein guiding optical structures employ TIR to redirect light towards a preferred direction; and
U.S. Pat. No. 6,129,439 to Hou et al. discloses an illumination apparatus in which microprisms utilize TIR for light redirection.
Zimmerman et al. '218, Kuper et al. '355, and Hou et al. '439 describe the use of a prism structure having at least one curved side wall shaped to use TIR, including a side wall having a large number of small segments to effectively provide an arcuate shape. While these disclosures show the use of side wall curvature, however, no guidelines are provided for optimizing the actual curvature or dimensions that work best. Some “rule-of-thumb” suggestions are proposed for relative proportions that seem suitable for various applications. However, prism side walls having arbitrary curvature and dimensions may not improve the performance of a brightness enhancement article and may, instead, be detrimental for brightness.
Parabolic reflectors are known in various types of applications for collecting or transmitting electromagnetic energy along an axis. In room lighting applications, for example, parabolic reflectors, and reflectors whose shape approximates a parabolic shape, are positioned around a lamp or other light source to collect light and direct it outward, generally in one direction. For optimal parabolic reflection of light along an axis, the light source is positioned at a focal point for the parabolic reflector.
More efficient light concentrators, such as compound parabolic concentrators (CPC) have been used for collecting light in various applications, particularly for solar energy applications. For example, U.S. Pat. Nos. 4,002,499 and 4,003,638 (both to Winston) disclose the use of reflective parabolic concentrator elements for radiant energy collection. U.S. Pat. No. 6,384,320 (Chen) discloses the use of an array of reflective CPC devices used for a residential solar-power generation system. Light concentrators have also been used to support light sensing devices. For example, UK Patent Application GB 2 326 525 (Leonard) discloses the use of a reflective CPC array as a concentrator for obtaining light for a light sensor, such as a Charge-Coupled Device (CCD). CPC and similar structures have been exploited for collecting and sensing light in various applications, but not for achieving improved light distribution and redirection.
Brightness enhancement films for optical displays have largely been directed to improving brightness of a display over a range of angles. However, spatial uniformity is also important, as it helps to minimize “hot spots” in a display. Existing brightness enhancing films, in an effort to achieve higher brightness, often tend to compromise display uniformity, causing hot spots and other anomalies.
In spite of the concerted effort that has been expended for increasing display luminance, improvements are required, particularly where a high level of near-axis luminance is desired and where spatial uniformity is desirable in the displayed image.